ADD/DROP multiplexers (ADMs) are used in three major applications. The first is for time-division multiplexed (TDM) systems that can be made with electronic time-division multiplexed (TDM) circuits or implemented using optical techniques, namely optical time-division multiplexed (OTDM) circuits. The second application is for wavelength-division multiplexed (WDM) systems which are operated in optical systems incorporating optical fiber or free-space as the optical transmission medium. The third application is in space-division multiplexing (SDM) systems, where different channels are transmitted between the same source and destination, but the transmission is via alternate paths (e.g., multiple fibers in an optical network). In the prior art, additional dedicated switching apparatus was used to provide the reconfiguration ADM function. For example, in TDM systems, an electronic switch fabric is placed between the low-rate and fast-rate ports. In OTDM systems, an optical channel selector is placed between the low rate and fast-rate ports. In WDM systems, optical filters or switching fabrics can be used, or Mux/Demux of the wavelength channels is performed, the signals converted from optical-to-electrical-to-optical with a switching fabric included in the electrical circuitry. For the SDM systems, the optical signals are converted to electrical, switched electronically and then converted back to optics (all signals remaining at a low-rate).
There is a continuing need to reduce the number of components, size, and complexity of reconfigurable ADMs. What is desired is a lower-cost, higher-volume reconfigurable multiplexer/demultiplexer apparatus that can provide multiplexer/demultiplexer or ADD/DROP multiplexer functionalities.